纳米粒子大小效应x射线光电子能谱研究物理基础

1、纳米粒子大小效应x射线光电子能谱研究物理基础2OutlineIntroductionNanoparticles (NPs)Size matters: surface of NPsSize-dependent propertiesXPSBasic principleInformation depthSurface composition and chemistry Depth profilingNew challenges of XPS in nanostructuresIdeal nanostructured surface patternsPossible nanostructured su

2、rface patternsChallenges issuesPhotoelectron peak shape of thin layer materialsPhotoelectron emission from nanoparticlesSize dependent electronic states of nanoparticlesPhotoelectron emission intensity in NPsDimensional estimation of nanoparticlesAngle-resolved XPS of nanoparticlesIssuesMonte Carlo

3、simulation modelSimulation resultsBinding energy and related shift of core levelBinding energy shiftValence band spectraAuger parametersWagner plotsInitial- and final-states effectsConclusions and future work31. IntroductionNanoparticlesA nanoparticle is a microscopic particle whose size is measured

4、 in nanometres (nm). It is defined as a particle with at least one dimension 100nm. Nanoparticles made of semiconducting material may also be labeled quantum dots if they are small enough (typically sub 10nm) that quantization of electronic energy levels occurs. Nanoparticles are of great scientific

5、 interest as they are effectively a bridge between bulk materials and atomic or molecular structures. A bulk material should have constant physical properties regardless of its size, but at the nano-scale this is often not the case. Size-dependent properties are observed such as quantum confinement

6、in semiconductor particles, surface plasmon resonance in some metal particles and superparamagnetism in magnetic materials. Semi-solid and soft nanoparticles have been manufactured. A prototype nanoparticle of semi-solid nature is the liposome nanoparticles have already been used for commercial purp

7、oses. Refrigerators and washing machines that are able to release silver nanoparticles enable the machines to kill micro-organisms, as the silver nanoparticles inhibit the respiration of the micro-organisms and suffocate them. This ensures that food will stay fresh for a very long time and clothes a

8、re cleaned thoroughly.Nanoparticle research is currently an area of intense scientific research, due to a wide variety of potential applications in biomedical, optical, and electronic fields.All these macro-properties of the nanoparticles are dependent on the their size 41.1 Size matters: surface in

9、 nanoparticlesEvolution of the dispersion F as a function of n for cubic clusters up to n = 100 (N = 106). The structure of the first four clusters is displayed E. Roduner, Nanoscopic Materials: Size-Dependent Phenomena. The Royal Society of Chemistry, Cambridge, 2006 Calculated mean coordination nu

10、mber NN as a function of inverse radius, represented by N1/3, for magnesium clusters of different symmetries (triangles: icosahedra, squares: decahedra, diamonds: hexagonal close packing). (Ref. 1, based on data in A. Khn, F. Weigend and R. Ahlrichs, Phys. Chem. Chem. Phys., 2001, 3, 711 Impact of s

11、urface atoms in NPs: (1) The fraction of atoms at the surface is called dispersion F; (2) Atoms at the surface have fewer direct neighbours than atoms in the bulk. Therefore, particles with a large fraction of atoms at the surface have a low mean coordination number (which is the number of nearest n

12、eighbours) Potential of the surface oxide reduction peak Ed of glassy carbon-supported Pt nanoparticles as a function of particle size. F. Maillard, S. Schreier, M. Hanzlik, E. R. Savinova, S. Weinkauf and U. Stimming, Phys. Chem. Chem. Phys., 2005, 7, 385 51.2 Size-dependent propertiesCalculated co

13、hesive energies of various size magnesium clusters in their most stable geometry as a function of N1/3. Left: Background-subtracted differential scanning calorimetry melting endotherms for indium confined in controlled pore glass (ac) and in Vycor samples (d) with different pore diameters. Note that

14、 the melting feature of the pore-confined material moves to lower temperatures and broadens as the pores get narrower. Right: Melting temperature as a function of pore diameter and inverse diameter. The broken line represents the bulk melting point a) Fluorescence of CdSeCdS coreshell nanoparticles

15、with a diameter of 1.7 nm (blue) up to 6 nm (red), giving evidence of the scaling of the semiconductor band gap with particle size. (Figure courtesy of H. Weller, University of Hamburg). b) Schematic representation of the size effect on the gap between the valence band (VB) and the conduction band (

16、CB) and the absorption (up arrow) and fluorescence (down arrow). Smaller particles have a wider band gap. . 61.2 X-ray Photoelectron Spectroscopy (XPS) X-ray Photoelectron Spectroscopy (XPS) , or ESCA - Electron Spectroscopy for Chemical AnalysisMajor components:X-ray Source High Vacuum EnvironmentE

17、lectron Energy Analyzer71.2.1 Basically principle XPS works by irradiating a sample material with soft x-rays causing core level electrons to be ejected. Surface information offered by XPS elements chemical status elemental composition electronic state depth analysisX-ray excite photoelectron and Au

18、ger electron processes81.2.2 XPS information depth Elastic Scattering corrections in AES and XPS. II, Estimating Attenuation Lengths and Conditions Required for their Valid Use in Overlayer/Substrate Experiments, by P.J.Cumpson and M.P. Seah, Quantitative Electron Spectroscopy of Surfaces:A standard

19、 Data Base for Electron Inelastic Mean Free Paths in Solids. By M.P.Seah and W.A. Dench An XPS surface analysis is not like slicing off a few nanometers of the sample surface and counting the atoms in the slice. The Beer-Lambert law tells us that the signal intensity from a layer of atoms buried a d

20、istance z beneath the surface is less than the signal intensity one would obtain from that layer had it been at the surface by a factor where is the photoelectron attenuation length and the photoemission angle. 95% of the information obtained by XPS comes from within three attenuation lengths of the

21、 surface , say 1-10nm91.2.3 Surface compositions and chemistry101. 2.4 Surface chemistryOxidation of PdOxidation of Mo with different chemical statesSpectra deconvolution may be needed in order to quantitative estimate of chemical states in most case due instrument resolution limitation111.2.5 XPS d

22、epth profiling: composition depth distributionAngle resolved XPSIon etching XPS122. New challenges of XPS in nanostructures 2.1 General nanostructured surface patterns(a)Thin layer(b)Island-like(c)Hemisphere(d)Sphere132.2 Possible nanostructures patterns 14 XPS: challenge issues in nanostructuresUni

23、form surface layer-Chemical states: related to charge-Exchange between atoms-Quantitative analysis can be carried byrelative sensitive factorsjjjiiSISICNew ChallengesNon-uniform surface(size effects)-Chemical states : Core level shift is not -only related to charge exchange between-atoms, but also t

24、o structural dimensions-Quantitative analysis?Conventional XPS153. Photoelectron peak shape of thin layer materialsThin layer position can causes different photoelectron scattering (peak shape), as found by D.R.Penn 30 years ago, and confirmed by S.Tougaard, as showing in Figures. Tougaard et al hav

25、e shown that the inelastic scattering background near the peak can be used to reliable obtain detailed information about the surface under investigation abcd1.1A50A20A30A50AD.R.Penn, Phys.Rev.Lett., 40(1978)568; S. Tougaard, Surf.Interf.Anal. 26(1996)249; J.Vac.Sci.Technol., A14(1996)1415; J.Vac.Sci

26、.Technol., B13(1995)949164. Photoelectron emission from nanoparticles Size dependent electronic states suggested that binding energy may affected by dimension of nanoparticles!P. P. Edwards, R. L. Johnston and C. N. R. Rao, On the Size-Induced Metal-Insulator Transition in Clusters and Small Particl

27、es, in Metal Clusters in Chemistry, Vol. 3, ed. P. Braunstein, L. A. Oro, P. R. Raithby, Wiley, Weinheim, 1999 4.1 Size dependent electronic states of nanoparticles174.2 Photoelectron emission intensity in NPsFor rectangle or square shape, photoelectron emission intensity from a nanoparticles, with

28、a narrow size distribution, following as)exp(1 0cCCdIIThen the intensity ratio from a nanoparticle with different mean escape depth , can be expressed intoIc: intensity: surface coverage of the nanoparticlesd: mean dimension of the nanoparticles,: photoelectron mean escape depth in the nanoparticles

29、 : photoelectron peak intensity from bulk materials0CI)exp(1)exp(121020121ccccccddIIII18For a sphere particles2/ 1 1)/(2()/(/2230dcceddIIG.K. Wertheim and S.B. DiCenzo, Phys.Rev. B 37(1988) 844 For a sphere particles with shell2/ 1) 1/2()/()/(),(/223/rderreddrrdI11)(321221ddRwhereD.-Q. Yang, J.-N. G

30、illet, M. Meunier and E. Sacher, J. Appl. Phys. 97(2005)024303dr195. Dimensional estimation of nanoparticles2/ 1) 12()(),(/223iriiiierrrI),(),(21rIrIRThe ratio of two photoelectron emission intensities from a spherical nanoparticle,having different escape depths , can be deduced fromThe dimension r

31、of the nanoparticles is only related the ratio of two photoelectrons from a nanoparticles.D.-Q. Yang, M. Meunier and E. Sacher, Appl. Surf. Sci., 173 (2001) 134and051015202530354081216202428 RNominal Cu thickness (A)The ratio of Cu2p3 to Cu3d as Cu mass thickness206 Angle-resolved XPS of nanoparticl

32、es010203040506070801.01.4(a) 3 , 0.1 /sec 3 , 0.02 /sec 8 , 0.007 /sec 8 , 0.2 /sec 8 , 0.15 /sec 8 , 0.008 /secCu2p3/2 Cu2p3/2 peak intensity (normalized)010203040506070801.71.8(b) 3 , 0.1 /sec 3 , 0.02 /sec 8 , 0.007 /sec 8 , 0.15 /sec 32 , 0.2

33、5 /secCu3d Cu 3d peak intensity (Normalized)216.2 Monte Carlo simulation model90, 0,)/21 (121/22forreIrsphereVdIIIdVzyxdIdI00/,expSLAExponential attenuation along the path d in the direction of observation. Straight Line Approximationxyzdv dMonte Carlo simulation modelStraight line approximation met

34、hod has been used for the simulation of photoelectron emissionK.N. Piyakis, D.-Q. Yang, and E. Sacher, Surf. Sci., 536 (2003) 139 22Monte Carlo simulation results2.32.501020304050607080AngleRelative Intensity0.1110Radius=5(0) Distance=2(0)/RRDRRD

35、2.32.501020304050607080AngleRelative Intensity0.11.010Radius=5(1) Distance=2(0)/RDR1DR2R2.32.501020304050607080AngleRelative Intensity0.11.010Radius=5(3) Distance=2(0)/RDR1DR2R3Effect of on the photoelectron emission2.32.501020304050607080AngleR

36、elative Intensity0.11.010Radius=5(3) Distance=2(2)/RD1R1D2R2R3The Monte Carlo simulations show that: the unexpected photoelectron emission behavior can be explained by different size distribution of the nanoparticles, separation and the ratio of the mean size to .237. Binding energy and related core

37、 level shift of NPs: Binding energy shift of core level Experimental resultsM.G.Mason, Phys.Rev.B27(1983)748; G.K.Wertheim and S.B. DiCenzo, .B37(1988)84424H.G.Boyen et al, Phys.Rev.Lett., 94(2005)01680425Size effects on the core level shifts: evaporated Cu nanoparticles on (a) HOPG and (b) BCB afte

38、r different surface treatment. Untreated HOPG and BCB surface gives very weak interaction with Cu, resulting larger NPs formation, Ar plasma (or beam) resulting in surface creates high density defects (free radicals) and N2 plasma treatment results in NHx groups formation.Fig(b) suggested that there

39、 may have addition contribution from surface chemistry besides size-dependent Cu NPs effects on core level binding shift.D.-Q. Yang and E.Sacher, . 195(2002) 187 26Major explanation of NPs binding energy shifting in XPS1. Initial effects proposed by Mason, due to surface atomic coordination number r

40、educing in NPs surface2. Charging effects (final-state effects) of residing positive charge on a NP, proposed by Wertheim et al, due to photoelectron emission from a NP and leaving a positive charge, causing core level shifts to high binding energy side.3. Both theoretical models indicate the bindin

41、g energy shifting is proportional to 1/d, d is NPs size4. Peak width of core level is increasing with NPs size decreasing, and it is difficult to understanding by charge residing model277.2. XPS Valence band spectra of NPsExperimental evidence:Valence electron peak shifts to low binding energy side

42、as NPs size increasePeak width decreases as NPs size decreasePossible explanation:Size-dependent surface electronic states induced valence band changes287.3 Auger parameters051015202530351849.01849.51850.01850.51851.01851.5 evapotation,untreated HOPG evaporation, Ar+-treated HOPG sputtering, untreat

43、ed HOPGAuger parameter (eV)Nominal Cu thickness (A)Auger parameter as a function of initial thin film deposition, which is used to evaluate initial thin film growth. Figure shows that different substrate have different contributions to the Auger parameters. There is no any affects of surface chargin

44、g in XPS using Auger parameters)()2(5 , 45 , 432/3MMLEpCuEKBThe Auger parameter has been used as an empirical, or “fingerprinting”, tool to characterize the chemical states of the elements in cases where charging of the sample or small shifts in core binding energies present problems.297.4 Wagner pl

45、otsWagner plot is also called a chemical state plot, which uses both photoelectron and x-ray excited Auger lines, increase the utility of XPS for identifying chemical statesKBEE 307. 5 Initial- and final-state effects of NPs in XPSXPS core level binding shift: is the change of core eigenvalue (initi

46、al state effects), Ef is change of final state relaxation energy, and EC is Coulomb energy, which originates from core-electron emission, leaving a cluster with unit positive charge that, in a free spherical metallic cluster, will appear as surface charge, resulting in Coulomb energy of e2/2r .Auger

47、 parameter is defined asWagner plotEk is photoelectron kinetic energyCfiBEEE)()2(5 , 45 , 432/3MMLEpCuEKBKBEE Core level binding shift is, in general, related to atomic chemical change. However, it can be affected by (i) surface charge, (ii) weak electron exchange, et al.The Auger parameter has been

48、 used as an empirical, or “fingerprinting”, tool to characterize the chemical states of the elements in cases where charging of the sample or small shifts in core binding energies present problems. Wagner plot is also called a chemical state plot, which uses both photoelectron and x-ray excited Auge

49、r lines, increase the utility of XPS for identifying chemical states.We use evaporated Cu nanoparticles and HOPG and BCB substrates to identify binding energy shifts, Auger parameters and Wagner plots due to relatively weak interactions between nanoparticle and substrate, to evaluate the size effect

50、s of nanoparticles on these parameters.310.020.040.060.01/d (A-1)Initial-state contribution to EB(Cu 2p3/2) (eV)-0.14-0.12-0.10-0.08-0.06-0.04-0.020.00Final-state contribution to EB(Cu 2p3/2) ) (eV)Cu NPs supported on HOPG after different surface treatment HOPGCyclote